THE MEANING OF PERCEPTION 175 



Are the changes that occur in the cells of the auditory centre 

 heard ? They may or they may not. If a gramophone starts 

 to play in the house next door just as a man is going to bed he 

 will certainly hear it ! But a man who is intent on some mental 

 work may not hear a knock at the door, while someone who is 

 with him but is not busy may hear it. And, of course, while he 

 walks through a busy street his eyes and ears receive a multitude 

 of stimuli, and, no doubt, transmit these to his brain, but he is 

 not aware of a large fraction of all this stimulation. Sensation, 

 then, may or may not be accompanied by changes of conscious- 

 ness. Now there is a stimulus leading to sensation, but there is 

 no " awareness," and, again, when the same stimulus is repeated, 

 we hear or see. Why in one case and not in the other ? 



There is a " psycho-physical " law which attempts to relate 

 together the intensity of a stimulus that can be measured and 

 the intensity of the feeling of awareness that a sense organ is 

 being stimulated. Let the stimulus be a weight placed on the 

 hand held out straight, and let the subject be asked to say when 

 he feels the increase of weight, as (unknown to him) increments 

 of mass are added to the latter. Say that the weight is P, and 

 let small mcrements'AP, 2AP, SAP, etc., be added to P ; the 

 subject will not recognise an addition to the weight until this 

 addition has become a certain fraction of the original weight. 

 For instance, he will not recognise that P+ AP is greater than P, 

 nor that P-J-2AP is greater. He witt recognise that P-J-3AP is 

 greater than P, though he will not recognise that P+3AP is 

 greater than P+2AP. Suppose, now, that there is a strict 

 determinism, and that every perception is a function of its 

 stimulus; then it is clear that /(P)=/(P-hAP)=/(P+2AP); 

 also /(P)</(P+3AP); and it also follows that P=P-fAP, 

 P+AP=P+2AP, P+2AP=P+3AP, and that P<P+3AP; 

 wherefore a thing which is equal to another thing is also less 

 than it ! This relation of the intensity of a stimulus to the 

 dependent perception is perhaps the only case in psychology 

 where (after taking some liberties with mathematics) we can 

 make a differential equation: 



dP 



and clearly we are led to a logical contradiction when we do so. 

 It is evident, then, (1) that a certain physical stimulus may 



